Grounded electrical systems have a low impedance connection between a grounded current carrying conductor (often referred to as a ‘neutral’) and a ground reference. This ground reference is often connected to earth in stationary system and chassis in vehicle systems. Should there be a fault (electrical connection) from one of the non-grounded conductors and another component of the electrical system at ground potential a fault current will flow back to the neutral through the low impedance connection. This fault current may then cause an over current condition on the non-grounded conductor causing over current protection (such as a fuse, circuit breaker, contactor) to open and remove power to the non-grounded conductor.
In certain applications it is desirable to reduce the fault current when current carrying conductors of an electrical system come into electrical contact with ground. One approach is to have an ungrounded system where none of the conductors are bonded to ground. In such cases, if any of the conductors fault to ground there will be no fault current flow, however this condition may not be evident and a second fault would then cause significant ground current to flow.
Prior art high impedance grounding systems such as illustrated in FIG. 1 are a mix between the grounded and ungrounded system. These systems have a resistance and or inductance between the neutral and ground references which limits fault current to levels which may be applied indefinitely. For example, companies make insulation monitoring devices which have hundreds of kilo Ohm's of resistance between a neutral reference and a ground reference. They can then measure the leakage current across this resistance. Some active devices send pulses with high frequency content across the high impedance (at line frequency, lower impedance at frequencies contained in the pulses) neutral to ground connection to measure system leakage. A disadvantage of high impedance systems is that they may be intolerant to small levels of leakage which would otherwise be acceptable. Systems that work by sending pulses through a higher frequency lower impedance path may take a significant amount of time to determine the level of electrical isolation between the ground and the system neutral.
Low impedance grounding systems are designed to allow for a significant, but limited, fault current to flow through the ground to neutral connection using some impedance to limit the current. On AC systems, the impedance may be made from inductive components with or without resistance. On DC systems, resistance is used for this current limited connection.
Typically, low impedance grounding systems are not designed to operate with the low impedance bonding path in place during a fault. These systems may clear the fault by:                a. Opening over current protection devices on the supply side as with a grounded system. This is typical in a utility system with impedance grounding on a neutral. While the protection device is open, power cannot flow.        b. Reducing power supplied to the electrical system until fault currents are reduced to acceptable levels. (See, for example, U.S. Pat. No. 6,829,556).        c. Opening the low impedance connection between the power system and ground. This is the standard action on locomotives with a ‘Ground Relay’ which opens to eliminate fault current flow through the ground resistor.        d. Opening the low impedance connection between the power system and ground, but leaving a second high impedance connection. This is done in (see for example, U.S. Pat. No. 5,867,358).        
It is notable that real power consumption (and therefore the power dissipation) in a resistor is much higher than an inductive line reactor in an AC system for a given voltage and steady state current. In DC systems, there is not an option to limit the steady state fault current through a ground connection using an inductive coupling. Because of this, low impedance systems typically use very large resistors or are designed so that the resistor path is removed from the circuit very quickly after current flow begins in order to limit the energy dissipated by the resistor to a level which will not cause a thermal failure of the resistor. In locomotive systems, the opening of this resistor is often accomplished through a set of contacts of the Ground Relay which opens when the current through the impedance connection from the power system to the ground (locomotive chassis) exceeds a threshold. At the same time, systems often remove power from the locomotive power system by disabling electrical operation of the generating machine on the locomotive or by disconnecting devices with the fault such as traction motors which have internal ground faults.